The present invention relates to an impedance element, in particular, relates to such an element which is used as a bead filter or a noise filter.
Conventionally, a bead filter is comprised of a cylindrical ferromagnetic hollow body, and a conductor line passing a hole of said hollow body. That bead filter is used as an inductor in a low frequency area, and as a resistor element in a high frequency area.
The impedance Z of a bead filter is shown by the following equation (1), in which the complex permeability (.mu.='+j.mu.") is introduced. EQU Z=(.mu..sub.0 LN.sup.2 /2.pi.)ln(O.sub.D /I.sub.D)(.omega.).mu."+j(.mu..sub.0 LN.sup.2 /2.pi.)ln(O.sub.D /I.sub.D)(.omega.).mu.'=R+j (1)
where .mu..sub.0 is permeability in vacuum (=4.pi..times.l0.sup.-7 H/m), .mu.' is real part of permeability; .mu." is imaginary part of permeability, ln is a natural logarithm, O.sub.D, I.sub.D and L are outer diameter, inner diameter, and length of a cylindrical hollow ferromagnetic body, respectively, N is number of turns (N=1 in case of a bead filter), Z is impedance, .omega. is angular frequency, R is the real part of the impedance, and X is the imaginary part of the impedance and j is an imaginary unit (j=.sqroot.-1). As apparent from the equation (1), a larger impedance is obtained when an inner diameter is small, and a length and an outer diameter are large.
However, a prior cylindrical structure of a bead filter has the disadvantage that the size of the device for large impedances must be large, and therefore, an impedance for each unit volume can not be large. In another word, in a given volume, it is sometimes difficult to provide a desired impedance.